Electromagnetic Waves and Optics
What this domain tests
This domain joins Maxwell's electromagnetic waves - production, speed, spectrum order - with ray and wave optics: mirrors, lenses, total internal reflection, interference, diffraction and polarisation.
Core rules
- c = 1/√(μ₀ε₀) = 3×10⁸ m/s; E₀/B₀ = c; E, B and the propagation direction are mutually perpendicular; no medium is required.
- Spectrum in order of increasing wavelength: gamma, X-ray, ultraviolet, visible, infrared, microwave, radio.
- Snell's law n₁ sin i = n₂ sin r; n = c/v; critical angle sin C = 1/n for dense-to-rare travel.
- Lens 1/v − 1/u = 1/f; mirror 1/v + 1/u = 1/f; power P = 1/f in metres, unit dioptre.
- Young's double slit fringe width β = λD/d; sustained interference needs coherent sources.
- Diffraction fringes are unequal in width; polarisation by reflection occurs at Brewster's angle, tan i_p = n.
Worked example
Light of 600 nm falls on slits 1.0 mm apart, with the screen 1.0 m away. Find the fringe width. β = λD/d = (600×10⁻⁹)(1.0)/(1.0×10⁻³) = 6.0×10⁻⁴ m = 0.60 mm. Convert nm to m and mm to m before dividing; skipping either conversion is the usual error.
Traps that cost marks
- Forgetting that on entering glass the frequency stays fixed while wavelength becomes λ/n.
- Mixing the different sign conventions for mirrors and lenses.
- Computing sin C with the calculator in radian mode.
- Treating diffraction minima as equally spaced like YDSE fringes.